1. Field of the Invention
The present invention relates to a laser beam scanning apparatus for use in image forming apparatus in laser printers, digital copying machines and the like, and more specifically relates to a laser beam scanning apparatus wherein a common optical system combines a plurality of laser beams.
2. Description of the Related Art
The image forming apparatus in printers, digital copiers and the like incorporate a laser beam scanning apparatus that modulates the luminance of a laser beam in accordance with the image signals, and exposes the laser beam on a photosensitive member, photographic film, screen or like exposure member.
The light source of a laser beam scanning apparatus uses a laser source capable of high-speed beam modulation and which can typically produce a very fine beam spot. While commonly known deflection means used to deflect the beam spot exposing the exposure member in the main scan direction typically include a rotatable polygonal mirror (multifaceted mirror), an oscillatable galvanomirror, a rotatable hologram and the like, the polygonal mirror is by far the most common deflection means inasmuch as the construction of such a mirror is much less complex than the others.
When the laser beam is deflected by the polygonal mirror, the scanning speed of the laser beam is proportional to the rotational speed of the polygonal mirror, such that the polygonal mirror may be rotated at high speed for high-speed scanning. In regard to the aforesaid high-speed scanning operation, the problems of motor durability and limitations of the material of the polygonal mirror must be overcome.
High-speed printers in particular use a plurality of laser beams modulated by respectively different image signals, such that the laser beams are simultaneously deflected so as to actually accomplish several high-speed scans by simultaneously scanning a plurality of parts on the exposure member.
On the other hand, the scanning apparatus provided in full-color printers and two-color (for example, black and red) printers expose two or more laser beams modulated by image signals corresponding to the images of the respective colors onto different parts of a single photosensitive member, or expose a plurality of photosensitive members by the respective laser beams so as to form a multicolor image.
The aforesaid type of scanning apparatus using a plurality of laser beams are typically constructed to commonly use a polygonal mirror and a lens so as to simplify construction and reduce cost, so that the plurality of laser beams are simultaneously deflected by the single polygonal mirror and projected by the single lens. Further, beam sensors are generally provided near the exposure member to determine the timing for the start of image modulation of the laser source. The laser beams deflected by the polygonal mirror passes through the beam sensors and scan the photosensitive member (exposure member).
An example of a laser beam scanning apparatus constructed so as to deflect a plurality of laser beams by means of a single polygonal mirror is an apparatus using a plurality of laser beams having respectively different characteristics (wavelength, the direction of the polarized and the like) which are combined to form a single beam that is deflected in the main scanning direction by a single deflection means, and thereafter split into a plurality of laser beams based their respective original characteristics, and each of the separated laser beams irradiate a different part of the exposure member.
For example, Japanese Unexamined Patent Application No. 60-201319 discloses a laser scanning apparatus for use in a color printer wherein the laser scanning apparatus first synthesizes or composes and then splits three laser beams having different wavelengths.
Disadvantages inherent to the previously described apparatus are described hereinafter. A first disadvantage is that the length of the separation mirror must be longer than the main scan direction of the exposure member so that the beam sensor can detect the beam because the beam sensor is disposed behind the separation mirror used to reseparate the plurality of deflected laser beam and guide the beams to the photosensitive member. Therefore the separation mirror is longer than necessary, and the overall size of the apparatus itself is enlarged.
A further disadvantage is that the wiring of the aforesaid apparatus is quite complex inasmuch as the beam sensor is arranged so as to be in a direction that is removed from the contro circuits that execute laser source illumination control.
A second disadvantage arises from the use of a dichroic mirror having a partially reflecting film that allows laser beams having certain characteristics to pass therethrough while reflecting laser beams having other characteristics. After the plurality of laser beams have been combined in a single beam and deflected, the dichroic mirror is used as a beam splitter to reseparate the plurality of laser beams according to their respective original characteristics.
The dichroic mirror comprises optical glass or like transparent material, the surface of which is coated with a partially reflecting film consisting of a dielectric multi-ply layer or a metal and dielectric multi-ply layer. Dichroic mirrors possess selectivity regarding wavelengths and the directions of the polarized of laser beams that are transmitted or reflected.
However, even a dichroic mirror having a partial reflective film possessing excellent selectivity cannot accomplish complete laser beam separation, and invariably produces some mixing of characteristics of one laser beam with the characteristics of another beam.
Therefore, a further disadvantage arises when each separated laser beam irradiates a different part of the surface of the exposure member in image formation, and the image formed at a particular part of the exposure member has superimposed thereon the image of a different part, which, when developed, results in the so-called "ghost" phenomenon.